An imaging device, such as a printer or copier, typically creates images using combinations of four colors of marking agents or colorants, such as cyan, magenta, yellow and black (CMYK). The images are created based on image data which assigns at least one of the four colors and a numerical color intensity or input color value to each picture element or pixel in the image.
A problem is that, due to manufacturing variations, environmental conditions and usage histories, different imaging devices can output different intensities of color based on identical image data. The density of the toner laid down on the print medium determines the color intensity. The denser or thicker the toner is laid down on a white print medium such as paper, the less white is visible through the toner on the paper. Consequently, the denser the toner, the less the lightness of the toner color, and the greater the intensity of the toner color.
Because there is such variation in toner density laid down by different imaging devices based on identical image data, color intensities that are output by some imaging devices can be outside of an acceptable range. Thus, in order to ensure that each imaging device outputs color intensities that closely correspond to the color intensities specified by the image data, each imaging device should be individually calibrated to output appropriate color intensities.
Traditionally, calibrating printers (e.g. any apparatus, such as a digital copier, bookmaking machine, facsimile machine, multi-function machine, etc. which performs a print outputting function for any purpose) has proven to be a less-than-efficient task, particularly if large amounts of data (e.g. test patches) are used. Recently, scanners have been contemplated as an efficient mechanism to aid in the calibration process. Traditional printer calibration (that is, not using a scanner) typically has employed a densitometer or spectrophotometer, which includes an aperture typically around 5 mm in diameter, the reflected light that passes through that aperture is optically averaged by the device. Scanner based printer calibration has traditionally involved averaging the area in some region analogous to the aperture of a spectrophotometer or densitometer.
Scanner-based printer calibration requires scanning portions of printed pages, applying some image processing technique to obtain averages over small regions of the page, in analogy to the spectrophotometer aperture, and then converting the averaged signal to a device independent color space. To do this conversion, the scanner must itself be characterized. Calibration is a process that obtains the change required to the device signals required to restore the device behavior to its nominal state; characterization is a process that obtains the conversion from device signals to device independent signals in some standardized color space.
Scanner characterization and scanner-based printer calibration usually involve scanning patches and then averaging the pixels in the central portion of the patch. Averaging reduces noise and emulates the effect of the spectrophotometer aperture; using only the central portion avoids any stray colorant from adjacent patches, as well as providing some protection against misregistration errors (that is, failure to locate the exact boundaries of the patch). However, it is possible to do better than emulating the averaging performed by a spectrophotometer with a fixed aperture, when processing a scanned image of printed patches. Using the methods of the presently described embodiments, smaller patches may be averaged, with greater precision (less noise) in the averages.